1. Field of the Invention
The present invention generally relates to fiber optic communication. More particularly, the present invention relates to a system and method for implementing a transimpedance amplifier (xe2x80x9cTIAxe2x80x9d) that is used in processing fiber optic communication data and capable of operating at and beyond 2.5 Gigabits-per-second (xe2x80x9cGb/sxe2x80x9d).
2. Discussion of the Related Art
Optical signals are increasingly being used to communicate between electronic processing devices. As a result, low-cost, high-performance fiber optic interface systems are becoming increasingly necessary. One integral component for fiber optic communication systems is a TIA. TIAs are commonly used for providing a voltage signal proportional to a current signal and are normally implemented by providing a feedback resistor across the input and output nodes of an operational amplifier. One use of a TIA is to convert an input current signal, which is indicative of an optical signal from a fiber optic line, into an output voltage signal.
In optical receiver modules, TIAs generally function as low-noise pre-amplifiers, which largely determines the overall performance of the optical module. In the past, because of the wide bandwidth and the high gain necessary for sensitive data links, TIAs have been implemented with bipolar and gallium arsenide (xe2x80x9cGaAsxe2x80x9d) metal-semiconductor field effect transistors (xe2x80x9cMESFETxe2x80x9d). These implementations result in high speed processing, but they can be costly and lack high-yield manufacturability.
Recently, metal-oxide-semiconductor (xe2x80x9cMOSxe2x80x9d) technology has become popular for the design of TIAs because of its low cost and high-yield manufacturability. However, using a single MOS gain stage fails to provide enough gain for multi-gigabit operation, because MOS transistors have a lower transconductance than bipolar transistors. Hence, successful high-speed MOS implementations have relied on multiple gain stages, while using various degeneration methods to control the stability of the TIA. One prior art design (xe2x80x9cDesign #1xe2x80x9d) is an n-channel metal-oxide-semiconductor (xe2x80x9cNMOSxe2x80x9d) TIA, having three common-source NMOS amplifiers with depletion mode loads constituting the open loop voltage amplifier. A local feedback is applied around the second gain stage to degenerate the gain. A similar second prior art design (xe2x80x9cDesign #2xe2x80x9d) utilizes three complementary metal-oxide-semiconductor (xe2x80x9cCMOSxe2x80x9d) amplifiers with diode-connected NMOS devices to control gain for the inverting CMOS amplifiers. However, neither Design #1 nor Design #2 is able to achieve an operating speed of 2.5 Gb/s. Design #1 operates at 1.0 Gb/s and 1.5 Gb/s, and Design #2 operates between 50 Megabits/second (xe2x80x9cMb/sxe2x80x9d) and 240 Mb/s. Moreover, Design #2 uses multi-gain stages to increase bandwidth, but no degeneration scheme is employed to control phase margin. Furthermore, in Design #2 only a large feedback resistor is used to control stability, resulting in a degraded overall bandwidth performance.
In an optical receiver module, an optical signal emerging from the fiber is converted to an electrical current by a photodiode, which is then amplified by a TIA. Input power incident on the photodiode preceding the TIA can vary greatly. Sometimes the variation can lead to a current input as large as 2 mA. For good noise performance, the TIA should have a transimpedance gain higher than 1 kxcexa9. However, a large current input can saturate the amplifier, leading to pulse-width distortion and bit errors. To alleviate this problem, numerous gain control techniques have been employed. In a third prior art design (xe2x80x9cDesign #3xe2x80x9d), U.S. Pat. No. 5,532,471, transimpedance feedback gain is lowered as a function of input drive. In Design #3, two circuit elements (one p-channel metal-oxide-semiconductor (xe2x80x9cPMOSxe2x80x9d) and one NMOS) are used to control the open loop gain, while a third element (NMOS) controls the transimpedance gain. Therefore, three elements are utilized to operate an automatic gain control (xe2x80x9cAGCxe2x80x9d) scheme, leading to a more complicated structure. Moreover, the use of both NMOS and PMOS to control the open loop gain can lead to additional process variations, because NMOS and PMOS device characteristics do not necessarily track each other.
In the TIA system disclosed in another prior art design (xe2x80x9cDesign #4xe2x80x9d), U.S. Pat. No. 5,602,510, the gain control drive that lowers the transimpedance gain is generated directly from the output of the TIA itself, which may lead to an ambiguous and process-dependent threshold for the activation of the gain control mechanism. This configuration may cause degraded sensitivity if gain control is turned on prematurely. Therefore, there is a need for a system and method that provide a more robust transimpedance gain control mechanism with a process independent threshold.
The use of clamping diodes as a gain control technique is employed in a fifth prior art MESFET design (xe2x80x9cDesign #5xe2x80x9d), U.S. Pat. No. 5,646,573. However, such designs are not very amenable to a CMOS implementation, due to the lack of high-speed diodes in CMOS technology. Therefore, there is a need to provide a system and method for implementing a TIA that accommodates better than 2 mA of current and is capable of operating at 2.5 Gb/s and beyond.